4o8 NATURAL SCIENCE. august. 



species is accompanied by sufficient variation among the species of a 

 genus to allow of the determination not only of the genus but also, in 

 the great majority of cases, of the species. 



The author proceeds to discuss, in the order of their importance, 

 the various characteristics presented by ligneous plants. 



I. The determination of the phyllotaxy, as indicated by the leaf- 

 scars. " It so happens that a classification of shrubs and trees into 

 those with alternate, spiral, and opposite or whorled leaves gives 

 three fairly equal sets, so that this determination at once excludes 

 a large number." In the case of rarer phyllotaxies, e.g., one-third 

 [SpivcBa opulifolia) or two-thirds [Catalpa speciosa), the identification 

 becomes still easier. 



II. The form of the leaf-scar and the arrangement of the 

 vascular bundles seen as dots on the exposed surface. The arrange- 

 ment may be circular {Moms rubra, Rhus avomatica, &c.), semicircular 

 (species of Fvaxinus), or horseshoe-shaped {Ptelea trifoliata, and also 

 species of Fraxinus), varying usually with the shape of the scar. Often, 

 however, the bundles form distinct sets, as in Ailanthus glandulosa, or 

 species oijuglans, with a more or less heart-shaped petiole, where we 

 find lunate sets. In other heart-shaped scars, and in most lunate 

 ones, the bundles form small circular areas, and the number of areas 

 in the fully-developed scar is fairly constant. Thus Uhnus fulva, 

 SpircBa opulifolia, Hamamelis vivginica, and others have three sets, while 

 Rhus toxicodendron, and species oi Msculiis, &c., have five. 



III. The character of the winter buds. These may be concealed 

 in the substance of the scar, or so covered by its anterior end that 

 their development towards spring causes more or less splitting of the 

 scar, as in Robinia pseudacacia. Sometimes they are sunk in the bark 

 of the twigs, as in Ptelea trifoliata. Prominently-developed scaly buds 

 can be classified into those showing exteriorly only one or two scales, 

 or a glimpse of a third or fourth {Liriodendron tulipifera, Rhus glabra, 

 Tilia americana, &c.), and those with four or more scales exposed. The 

 latter class can be further subdivided into those with terminal buds 

 much larger than the lateral (species of Fraxinus, Juglans, &c.), and 

 those where such a difference is not particularly marked. 



IV. The mode of termination of the branches. In the great 

 majority of cases terminal scaly buds are always present, but some- 

 times the green tips of the new twigs are cast off each spring, and 

 the place of the terminal bud is accordingly marked by a scar in the 

 winter twigs, as in Tilia americana and Ailanthus glandulosa. In other 

 species the tip of the branch shrivels up early, but is not cast off, the 

 shrivelled tip remaining through the winter {Diospyros virginiana, 

 and species of lilac). 



V. The presence or absence of stipules, as indicated by their 

 scars. As the stipules, when present, usually fall early in spring, a 

 little practice is required to easily recognise their indistinct scars, 

 which may more or less encircle the stem, as in the Tulip-tree, or as 



